A blue ray optical disk system has been developed as the next generation digital video disk (DVD) system. In the system, the recording capacity of an optical disk is 27 GB, which is a large increase in contrast to 4.7 GB of the conventional optical disk. In addition, there is a forecast for recording 50 GB or more by using a method of double layer recording. Since the large capacity of the blue ray system is realized by using a blue ray semiconductor laser, pick up objective lenses are required to focus the ray on a disk surface for reading and writing signals. There is a large difficulty in realizing the high quality objective lenses for focusing the blue ray using plastic material, because the lenses for focusing blue ray require quality several times higher than the objective lens quality for the conventional DVD system. In addition, the plastic lenses show focusing quality changes with temperature or environmental condition. Furthermore, the requirement for the blue ray optical disk system to satisfy compatibility with the conventional DVD systems makes the development of the optical system further difficult.
Since light sources used for the conventional DVD systems are red light semiconductor lasers, the optical system is required to operate to show high focusing performance for both red and blue ray of writing and reading of both conventional DVD signals and blue ray signals. To meet the requirement, for example, a method of using two lenses is known in which good performances are kept for both lights. In this method, a lens manufactured by a glass molding method is applied as the lens that contributes largely for focusing in combination with another plastic lens having diffraction grooves for changing its effect depending on the difference of the light sources. However, it is difficult to obtain high system performance using this method, because extremely high-speed driving performance is required for the objective lens.
Another method for realizing the objective lens is to form diffraction grooves on an aspherical glass lens for allowing functions of the plastic lens also. Then an objective lens that works as blue ray focusing lens compatible with the conventional DVD can be obtained by using a single glass lens having fine groove structure (Refer to Reference 1, for example).
The manufacturing of the glass lens in this type is desirable to be conducted by a molding method using a heat-resistant molding die to ensure its productivity. For this reason, it is a principal issue to develop a technology of forming a glass lens having fine structure using a heat resistant material. The molding die is needed to form a fine structure of diffraction grooves on the die surface for molding and transferring an optical surface to a molded glass lens.
Known dies for molding plastic molded lenses are fabricated by methods of forming electroless nickel plating on surfaces of blank die bases and cutting the plated surface by a diamond ultra-fine machine. However, this method takes process time of from several days to one week or two weeks for the plating to obtain plating layers without residual strains. Therefore, it takes several weeks for manufacturing one die in total including the ultra fine cutting process after the plating.
Inventors of the present invention already achieved development of a new technology that facilitates manufacturing these dies using amorphous metal alloys having metallic glass characteristics. The technology was realized by forming metallic glass layer on a surface of a blank die base by a sputtering method instead of using electroless plating process and then ultra-fine cutting is applied. This technology uses the characteristics of metallic glass having crystallization temperature Tx and glass transition temperature Tg that softens like thick malt syrup at super-cooled liquid state at temperature range between Tx and Tg. Then, molded dies are manufactured by applying the molding process using a mother die. The dies manufactured by this molding process are named as “cloned dies”. Application of the dies improved product accuracy and quality, in addition to reducing manufacturing process time (References 2 and 3). Applying this method, fine structures such as diffraction grooves can precisely be formed on a replicated optical surface. Due to the amorphous structure of the alloy, the alloy has characteristics not found for conventional metallic material such as an excellent cutting performance permitting formation of fine structure and so on in addition to microscopic homogeneity of composition, large mechanical strength and chemical stability. These characteristics are advantageous to obtain the die characteristics required for high precision molding.
To apply the manufacturing technology of molding dies using amorphous metal alloy thin film that is successful for molding plastic lens to the technology of molding dies for manufacturing glass lenses, following big technical problem must be solved. In contrast to the molding temperature of plastic lenses of about 200 degrees centigrade, the temperature of molding glass lens is higher temperature of about 500 degrees centigrade even when the Tg of the glass material is low. The temperature is 800 degrees centigrade or more when the Tg of the glass material is high. Therefore, the thin film metallic glasses used successfully for molding plastic lenses cannot be applied for dies of molding glass lenses. Therefore, a new metallic die material applicable in such a temperature range is required. Furthermore, the die material is essentially required to be resistant to bonding formation with glass material. It must be noted that glass material for lens application is generally reactive to die material and easily form fusion bond with a molding die.
The problem is not limited to the problem of amorphous metal alloy but also problems for using nickel plating to molding dies of conventional plastic glass molding. Dies plated with nickel cannot be used as dies for molding glass lenses. For this reason, hard metal having heat-resistant characteristics and resistant to bonding to glass is applied as die for molding glass lenses. However, there are big disadvantages of the hard metal material of extremely hard for machining, and requiring much time and cost for machining. The hard metal material is not suited for forming fine structure because a machining process by grinder must be applied as cutting process, and so the hard metal material is very difficult to form fine structure such as diffraction groves on optical surface of the die.
For this reason, it is desirable as a resolution of the issues explained above to find out amorphous metal alloy material, and especially to find out metallic glass film material that softens like thick malt syrup at super-cooled liquid state between Tx and Tg as a molding die material for realizing die for molded glass lens. Then, the amorphous metal alloy applying to a die for molding glass lens is required to have Tg at least at 600 degrees centigrade or more.
Therefore, metallic glass having Tx and Tg at high temperature range is required. Examples of known metallic glass having Tx and Tg at high temperature range are as follows.
Metallic glass of amorphous Fe alloys that are prepared as bulk metallic glass other than prepared as thin film plate or film and available as magnetic materials are described in patent literature 4. In this literature, amorphous Fe alloys containing semi metal elements P, C, B, Ge and so on, and metal elements Al, Ga, Sn and so on having temperature interval ΔT (where ΔT=Tx−Tg) of 40 K or more are disclosed. Tg values of these amorphous alloys are between 500 to 600 degrees centigrade.
In patent literature 5, amorphous Fe alloys expressed by Fe100-x-yMxBy where M=Zr, Nb, Ta, Hf, Mo, Ti, V or Cr having temperature interval of 20 K or more are disclosed. Tx values of these amorphous alloys are 500 to 600 degrees centigrade.
In patent literature 6, soft magnetic amorphous Co alloys having super-cooled liquid temperature interval ΔT of 40 K or more, a reduced glass transition temperature Tg/Tx of 0.59 and low coercive force of 2.0 A/m are disclosed. These amorphous alloys are expressed by [Co1-n-(a+b)FenBaSib]100-xMx, where M is at least one of Zr, Nb, Ta, Hf, Mo, Ti, V, Cr, Pd or W. The Tx values of these amorphous alloys disclosed in the literature are 640 degrees centigrade or less.
Although metallic glassy state is obtained by heating these amorphous alloys, they do not have resistance to bonding formation with glass at the process of molding, corrosion resistance and heat all together. Therefore, it is needed to obtain a novel alloy material suitable to this purpose having resistance to bonding formation against glass material in the process of molding, corrosion resistance and heat resistance to realize a die for fine molding of the glass lenses explained above.
Pt metal has both excellent corrosion resistance and excellent heat stability, but hardness of Pt metal is not so large. Therefore, it is desirable to obtain Pt amorphous alloy having large hardness maintaining its machining characteristics.
Amorphous alloys having compositions of (Pt1-xNix)75P25 such as Pt60Ni15P25 are reported (non-patent literature 1). Although the alloys are Pt alloys and show properties suggesting metallic glassy state, the (Pt1-xNix)75P25 alloys have a problem in chemical stability because the alloys contain P. Noble metal base alloys of Pt—Cu—P alloys (where 50≦Pt≦75, 5≦Cu≦35, and 15≦P≦25) are disclosed (Reference 6). These alloys also have a problem in chemical stability despite that the alloys are Pt alloys. As other Pt alloys that show metallic glassy state, Pt alloys having Pt20Zr80 and Pt20Zr70Ni10 compositions are reported (non-patent literature 2). However, these alloys have a problem of low heat resistance because Tg values of these alloys are low.
As explained above, none of the known Pt alloys fit to material of die for high precision molding of glass lenses and so on. These results dismiss the possibility of applying the alloys as alloy material of the die for fine molding of the glass lenses and so on.
[Reference 1] JP-A 2005-209321 (KOKAI)
[Reference 2] JP-A 2003-154529 (KOKAI)
[Reference 3] JP-A 2003-160343 (KOKAI)
[Reference 4] JP-A H08-333660 (KOKAI)
[Reference 5] JP-A 2000-256812 (KOKAI)
[Reference 6] JP-A 2003-301247 (KOKAI)
[Reference 7] JP-A 2002-53918 (KOKAI)
[Non-Patent Literature 1] Journal of Non-crystalline Solids: Vol. 18, p. 157 (1975)
[Non-Patent Literature 2] 644th Materials Research Symposium Proceedings L.1.1, p. 1 (2001)